Water electrolysis system improving durability by preventing performance degradation inside water electrolysis stack

Abstract

Disclosed is a water electrolysis system that improves durability by preventing performance degradation inside a water electrolysis stack. According to the present invention, in order to reduce electrode degradation in a water electrolysis unit cell, which can frequently occur in the starting and stopping stages of a process for producing hydrogen from the water electrolysis system, power of a constant current is supplied to the water electrolysis stack and electrolyte circulating water is heated while being circulated in the water electrolysis stack in the starting stage of the water electrolysis system. Also, when performing a stopping process, power of a constant current is supplied to the water electrolysis stack and electrolyte circulating water is cooled while being circulated in the water electrolysis stack. Accordingly, it is possible to improve durability by preventing performance degradation inside the water electrolysis stack.

1 . A water electrolysis system of which durability is improved by preventing performance degradation of an inner portion of a water electrolysis stack, the water electrolysis system comprising:
 a water electrolysis stack  101  having a structure in which a plurality of unit cells and separating plates are stacked in series and configured to produce hydrogen and oxygen through a water electrolysis reaction by electric energy;   a cell voltage reducer  102  which detects that a cell voltage of the water electrolysis stack  101  is lowered from a predetermined cell operating voltage in real time;   a current variable converter  103  which supplies the electric energy to the water electrolysis stack  101 ;   a breaker  103 - 1  which is provided at one side of the current variable converter  103  and blocks current from flowing to a back-pressure regulator  120  and the water electrolysis stack  101 ;   a circulating water tank  104  which is provided at one side of the water electrolysis stack  101  and supplies circulating water thereto;   a liquid pump  105  positioned on an oxygen generation line on which oxygen is generated at an anode (OER; oxygen evolution reaction) electrode of the water electrolysis stack and configured to circulate circulating water, which is an electrolyte;   a pressure sensor  106  provided at front end of the water electrolysis stack  101 ;   a flow sensor  107  provided at one side of the pressure sensor  106  and configured to detect a flow rate of the circulating water;   a temperature sensor  108  which is provided at rear end of the water electrolysis stack  101  to monitor an increase in a temperature of the circulating water at the rear end of the water electrolysis stack  101  due to a mixing phenomenon of hydrogen and oxygen when a pinhole is generated in an ion-exchange membrane;   an air-cooled heat exchanger  109  which maintains a constant temperature of the circulating water tank  104 ;   a water electrolysis system controller which detects a dangerous state due to the mixing phenomenon of hydrogen and oxygen;   a circulating water auxiliary tank  110  which supplies as much water as an amount of water consumed in the circulating water tank  104  in real time;   a cooling heat exchanger  114  which cools hydrogen produced at a cathode (HER) electrode of the water electrolysis stack  101 ;   a pressurizer which is provided on a hydrogen production line and prevents water coming through the ion-exchange membrane from a side of the anode electrode toward the cathode electrode;   a check valve which prevents water from flowing back through the ion-exchange membrane;   a water separator  112  which separates a small amount of moisture transferred with hydrogen;   an absorber  117  positioned at one side of the water separator  112 ;   a microfilter  118  which is provided at one side of the absorber  117  and allows high-purity hydrogen to be obtained with the absorber  117 ;   a pressure sensor  119  provided at rear end of the water electrolysis stack  101 ;   the back-pressure regulator  120  provided at one side of the pressure sensor  119  and adjusts a hydrogen production pressure; and   an explosion-proof oxygen sensor  121  which detects a concentration of oxygen in produced hydrogen gas,   wherein, in a start process of the water electrolysis system, the electrolyte circulating water of the water electrolysis stack is circulated while heated to a predetermined temperature, and power with a predetermined current is supplied to the water electrolysis stack until the temperature of the electrolyte circulating water reaches the predetermined temperature; and   in a shut down process of the water electrolysis system, power with a predetermined current is supplied to the water electrolysis stack, and the electrolyte circulating water is cooled to a predetermined temperature.   
     
     
         2 . The water electrolysis system of  claim 1 , wherein a current supplied to the water electrolysis stack is in a range of 10 mA/cm 2  to 50 mA/cm 2  based on an active area of the water electrolysis stack. 
     
     
         3 . The water electrolysis system of  claim 1 , wherein, in the shut down process of the water electrolysis system, a power supply is stopped in a region where a temperature of the electrolysis circulating water is 40° C. or less, and a pressure of a hydrogen generating part of the water electrolysis stack is higher than a pressure of an oxygen generating part of the water electrolysis stack;
 the cell voltage reducer lowers a voltage of the water electrolysis stack to a predetermined voltage; and 
 the circulation of the electrolyte circulating water is stopped for a predetermined time period. 
 
     
     
         4 . The water electrolysis system of  claim 3 , wherein a time period for which the circulation of the electrolyte circulating water is stopped is in a range of 10 seconds to 30 seconds. 
     
     
         5 . The water electrolysis system of  claim 3 , wherein a voltage causing the voltage of the water electrolysis stack to be lowered allows a voltage of the plurality of unit cells constituting the water electrolysis stack to be lowered to a range of 0.3 V to 0.5 V.